The bandwidth of ordinary amplifiers is typically limited by parasitic capacitances. As frequency increases, the impedance of such capacitances decreases, thereby tending to short-circuit the signal to ground or to other circuit nodes. A common approach to extend the bandwidth of amplifier circuits consists of adding inductors at judicious locations. When properly designed, the combination of such inductors and the surrounding parasitic capacitances results in an equivalent impedance retaining a substantial magnitude over a wider frequency range than the parasitic capacitances alone, thereby extending the amplifier bandwidth. Transmission lines constitute a particular example of this technique. Such lines have distributed capacitance to ground tending to short-circuit the signal, but the existence of distributed inductance along the line results in an overall impedance which is resistive at any frequency (at least in a somewhat idealized theoretical case).
Some amplifier circuits (known as “distributed amplifiers”) take advantage of this fact. Typically, they consist of multiple ordinary amplifier circuits connected in parallel through inductors (see e.g. D. M. Pozar, Microwave engineering, 2nd ed, John Wiley & Sons, 1998). The parasitic capacitances associated with the inputs and outputs of the ordinary amplifier circuits combine with the inductors to form transmission lines. Therefore, these parasitic capacitances do not contribute to restrict the bandwidth of the ordinary amplifier circuits. They are said to be “absorbed” by the transmission lines. As a result, distributed amplifiers typically achieve a much larger bandwidth than individual ordinary amplifier circuits used in isolation. However, this type of distributed amplifier suffers from some shortcomings. One of the problems is that the transmission lines connecting the ordinary amplifier circuits together unavoidably have losses due to the series resistance of the inductors. Such losses cause the input signal to wane as it travels along the transmission line connecting the inputs of the ordinary amplifier circuits, thereby limiting the number of amplifiers which can be put in parallel. A second shortcoming is that the overall gain of the distributed amplifier grows only lineary with the number of elementary amplifiers used in the structure. It would take a large number of elementary amplifiers to achieve a large gain.